THE BRITISH FRESH-WATER PLANARIANS. 5
that the intestinal branch on the left of the pharynx has from
16 to 19 lobes; the branch anterior to the pharynx has 10 to 15
pairs of lobes, and the right branch, like the left, has from 16 to
19 lobes.
The ultimate branches of the intestine have blind ends, con-
sequently undigested fragments of food are returned through the
mouth. The soluble waste products pass through the renal
excretory system, which is made up of thin elongated tubes, form-
ing a series of complicated loops, which pass along the body.
These tubes open to the surface of the body by a series of pores,
whose number and position, as well as the number of tubes,
vary in different species. The renal tubes end in tiny blind sacs,
at the bottom of each of which is a cell provided with a long
vibrating lash which projects into the sac. The rapid vibrations
of their cilium resemble the flickering of a flame; the whole
organ is therefore termed a "flame cell."
There are no special organs of respiration, that process being
apparently carried on by means of the skin.
Immediately behind the mouth, and between the posterior
branches of the intestine, the outlines of the genital organs
may be distinguished in mature individuals at certain seasons
of the year. The animals are hermaphrodite and when sexually
mature show, (a) male organs consisting of a penis (p., figs.
1, 9 and 10), to which is attached the vas deferens,2 and the
testes;2 and (b) female organs—the uterus (ut., figs. 1, 9 and 10),
connected with the ovaries by a long oviduct,2 and the vitelline
glands,2 which provide the yolk for the egg. Both male and
female organs open into a common chamber, namely, the atrium,
which opens externally by the genital pore (g.p.). Mutual copula-
tion between two individuals takes place. The ova are ferti-
lized by spermatozoa, which, in the case of Planaria torva, are
enclosed in a spermatophore.
The greater part of the surface of the body of a planarian is
covered with cilia by whose rhythmic movements the animal
glides in a highly characteristic manner. These cilia are attached
to epithelial cells, which contain a number of small rods, from 10μ
to 16μ in length, called rhabdites, arranged in the cells with their
long axes at right angles to the body surface. The rhabdites
are formed, not in the epithelial cells, but in a deeper layer of
2. These organs are omitted from Fig 1 in order to avoid overcrowding the diagram.